Interspecific Aggression is competition for resources among individuals of different species. This resource competition can become aggressive and is energetically costly and generally risky. Natural Selection should favor the competitor based on the overlap of competitive aggression among species.
A study by Frank R. Moore (1978) shows that mockingbirds in North Carolina are provoked by other frugivores which promotes aggression between the mockingbirds and competing species. Non-frugivores in the mockingbird territory were ignored and aggression was only seen between mockingbirds and species that directly threatened their resources.
Competition between predator and prey is the most common form of interspecific aggression. Though predation is not aggression, the aggressive protection of habitat, resources, and kin is considered interspecific aggression. 
- ↑ Moore, Frank R. (1978). Interspecific Aggression: Toward whom should a mockingbird be aggressive? Behavioral Ecology and Sociobiology: 3: 173-176. Retrieved from http://link.springer.com/article/10.1007%2FBF00294988
- ↑ Aggression. Retrieved from http://en.wikipedia.org/wiki/Aggression